Production of UHMWPE sheet materials

ABSTRACT

Wide panels of UHMWPE having exceptional flatness and surface quality are ram extruded through a slit die equipped with a plurality of individually adjustable cooling zones proximate the panel exit slit and transverse to the machine direction which cool the by a back pressure device and preferably a vertical movement restraint. The process is robust and can be run for long product campaigns.

This application is a continuation-in-part of U.S. application Ser. No.11/491,416 filed Jul. 21, 2006, U.S Ser. No. 11/491,356 filed Jul. 21,2006, and U.S. Ser. No. 11/491,361 filed Jul. 21, 2006, all pending,priority to all of which is hereby claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the production of sheets of ultrahigh molecular weight polyethylene in widths greater than about 40 cm,by ram extrusion, and to sheets produced thereby.

2. Background Art

Conventional polyethylene polymers with which many are familiar, i.e.low and high density polyethylene, are waxy solids not known forexceptional strength properties. Also, due to their relative softness,while generally lubricious, they are easily abraded. Ultra highmolecular weight polyethylene, “UHMWPE”, on the other hand, is anexceptionally strong and durable polymer totally unlike its poorerrelatives.

UHMWPE is prepared by special polymerization processes in which thepolyethylene chains acquire an exceptionally high molecular weight,typically having number average molecular weights of from 1.5×10⁶ to1×10⁷ daltons, supplied as a resin powder. UHMWPE also includes thosepolymers defined in ASTM D4020-05; D6712-01; and ISO 11542-2. Whilegenerally homopolymeric in nature, UHMWPE also includes copolymershaving limited amounts of other copolymerizable comonomers. In the caseof copolymer UHMWPE, the aforementioned ASTM and ISO requirements shouldbe met. It is their very long polymer chains which make these polymersunique. However, this same feature is also the cause of major processingproblems. While ordinary polyethylene can be melt extruded, evenpolyethylene of very high molecular weight, attempts to melt extrudeUHMWPE have been largely unsuccessful, despite much research in thisarea, and virtually all UHMWPE products are produced by compressionmolding or ram extrusion, as indicated by U.S. Pat. No. 5,286,576, whichindicates that processing methods applicable to conventionalthermoplastics, such as continuous extrusion, calendaring, and injectionmolding, are generally inapplicable to UHMWPE.

Thus, for example, U.S. Pat. No. 5,422,061 discloses the manufacture ofsliding members by screw extrusion of UHMWPE from the melt. However, forthe process to work, mixtures of UHMWPE, lower molecular weightpolyethylene (“PE”), and lubricants must be employed. Considerable lossin mechanical properties thus results due to the dilution of the UHMWPEwith lower molecular weight species. Further losses in properties arecaused by shear degradation in the extruder.

U.S. Pat. No. 5,399,308 discloses melt extrusion through a die whosefinal cross-section is considerably reduced as compared with thediameter of the extruder, and wherein a very low elongational velocitygradient is maintained. However, despite these requirements, only veryhigh molecular weight PE and UHMWPE in the lowest molecular weightrange, about 1.5×10⁶, are useful. U.S. Pat. No. 5,449,484 discloses ascrew geometry for a single screw extruder said to be useable with PEresins having molecular weights greater than 1×10⁶. However, largeprofiles cannot be made using such a melt extrusion process.

The problems associated with processing of UHMWPE are due in part to thegel-like nature of the polymer above its crystalline melt temperature,roughly 135° C.-150° C. While ordinary polyethylene is a viscous,molasses-consistency liquid at such temperatures, UHMWPE is a swollengel which has an extremely high viscosity, and which exerts exceptionalfrictional stress against the walls of extruders and the like. Theproblems associated with UHMWPE are described quite clearly by U.S. Pat.Nos. 3,883,631 and 3,887,319. For the reasons discussed therein, ramextrusion and compression molding have thus far been the only processesof producing UHMWPE products which are used extensively.

In compression molding, UHMWPE powder is introduced into a coffin-likemold of very thick section. A thick cover which fits within the moldcavity is then placed on top of the powder, and the whole assembly isheated to above the crystalline melt temperature while being compressedto very high pressure. The molds are then slowly and uniformly cooled,and the product, generally in the form of a thick slab, is demolded. Forthin stock, for example of 1 cm to 3 cm thickness, the thick slabs areskived or “planed” to produce thin stock. The skiving process requiresan extra process step, and can result in a product with end curl. As canbe seen, compression molding is a cost-intensive and labor intensivemethod of producing UHMWPE products. However, it is essentially the onlyprocess which can be used to make panels or sheets of large width, andis thus still a much used process.

A continuous process for the production of thin gauge UHMWPE sheet hasbeen used by Crown Plastics, Harrison, Ohio, U.S.A. In this process aroller belt press previously manufactured by Hoover Ball and Bearing Co.as the Lam-N-Hard laminator, and as described for use in wood laminationby Tarkow, et al., “Surface Densification of Wood,” FOREST PRODUCTSJOURNAL, 18(a): 104-107, is used to consolidate UHMWPE powder. However,the process thus far has been limited to thin sheets with a maximumthickness of 3-4 mm and relatively narrow widths. Only recently has a 24inch wide (0.6 m) sheet been produced by this method, and it is notbelieved to be possible to produce wider sheets due to the highpressures involved.

Ram extrusion is a unique process which is considerably different frommelt extrusion. Ram extrusion may be illustrated with reference to U.S.Pat. Nos. 3,883,631; 3,887,319; and 4,145,175. Despite the fact that theram extrusion technology disclosed in these references is more than 25years old, there has been only incremental change in ram extrusionprocesses since that time.

The overall process may be described in relation to FIG. 1 which showsschematically, in cross-section, a simple ram extrusion machine forproduction of a UHMWPE rod. The ram extrusion machine consists of verythick section steel member 2 having a through channel 3 into one end ofwhich is received ram 4. UHMWPE powder 5 flows gravitationally intochannel 3 from hopper 6. The ram then travels to the left, compressingthe powder in the channel, which is now die channel 7. This sequence isrepeated every few minutes. Die channel 7 is heated by heaters 8 whichsurround the die, and heats the resin particles to a relatively hightemperature, for example between 350° F. and 500° F. (177° C. and 260°C.). Temperatures in excess of 500° F. (260° C.) are generally avoided,since the polymer rapidly oxidizes above this temperature. Oxidizedpolymer exerts yet more friction with the die, and due to the oxidation,products have reduced physical characteristics. The ram exerts apressure up to several thousand lb/in², and consolidates the heated,gel-like particles of UHMWPE. The UHMWPE rod 9 exits the die at discreteintervals and at this stage the rod is hot, higher than the crystallinemelt temperature, and relatively translucent. However, at some distancefrom the die face, the polymer has crystallized to an opaque solid.

Ram extrusion has been used to produce round profiles of relativelylarge diameter, e.g. 300 mm, and also tubing, small profiles of complexshape, and “boards” having a width of up to about 660 mm, andthicknesses of, e.g. 100 mm. However, such boards are far from flat asproduced. If flat boards or thin stock are needed, the boards areskived. Because of the high friction within the die, and consequentlythe very high pressures involved, the ram, even though made of very highstrength steel, may buckle. This is particularly so in parts whosecross-section is quite asymmetric, and even more so in parts having asubstantially rectangular cross-section of high aspect ratio. Forexample, a ram for producing a board of 1 cm×30 cm rectangularcross-section may exhibit buckling, whereas a ram for a thicker board,for example 10 cm×30 cm, will have virtually no problem in this respectat all. Not only can buckling be destructive of the ram, but thedistorted ram may scrape the die walls, introducing metal particles intothe product and altering die geometry.

It would be highly desirable to employ ram extrusion to produce sheetsand panels of large width, for example 1 to 3 meters in width, and of arange of thicknesses, in particular, in standard thicknesses which canbe sold as is, without skiving. However, attempts to use the ramextrusion process to prepare such sheets and panels meeting thenecessary product standards have been largely futile. The failure ofothers in the past can be attributed to a number of factors. First, thenature of UHMWPE is such that there is considerable volume contractionupon both cooling and crystallization. Differential cooling orcrystallization generates internal stress, as does also differingdegrees of polymer orientation. In small profiles or even largerproducts which are relatively symmetrical, these problems are minimal,or are to a degree self-cancelling. However, in large widths, theseproblems manifest themselves as undesirable “product characteristics”such as thickness variations, bowing, warping, surface fracture, surfaceirregularities, “walking”, edge waviness, etc. The larger the width ofthe product, the more difficult is the control of these properties. Suchdefects would then require minimally, shaving of the surface andmachining to size. However, some defects, for example warp and bow, maybe impossible to remove.

Moreover, the ram extrusion apparatus itself also has severeshortcomings. The large top and bottom surface areas associated with aslit die, coupled with the large internal pressure, create forces whichare very difficult to control. A slit die of 1 cm height and 1 m width,and of 0.5 m length may experience a force of 2.1·10⁶ N or more on eachhalf of the die depending upon the internal pressure, which is alwayshigh. The restraining bolts in this case will have to bear almost4.4·10⁶ N of force. Even when such a die is constructed of high strengthstainless steel of, for example, a 10 cm thickness on each side, the diewill deflect so much due to the internal pressure that a board withpronounced greater thickness in the middle as opposed to the edgesresults. Dies of this size and construction will also rupture, unlesssupported by massive restraining structures located exterior to the die.

For all the above reasons, it has been considered impossible to producewide sheets and panels of a quality which is commercially acceptable, byram extrusion.

SUMMARY OF THE INVENTION

It has now been surprisingly discovered that flat panels of UHWMPE inwidths greater than 0.4 m and of substantially constant thickness can beproduced by ram extrusion through a slit die as a partially crystallinesolid, the slit die provided with a plurality of transversely positionedcooling zones on both the top and bottom of the die proximate the exitof the die, and followed by a back pressure device. It has further beensurprisingly discovered that a higher quality, flatter product isobtained if a die is provided which is sandwiched by a supportstructure, the die having a slit of non-uniform height when not underpressure, but in cooperation with flexure of the die and supportstructure under ram extrusion pressure, is distorted so as to provide asubstantially flat product. Preferably, the die as described is used inconjunction with the multiple cooling zones and back pressure device toprovide a stable and robust process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically the process of ram extrusion.

FIG. 2 is a perspective drawing of a die and support structure for oneembodiment of a ram extrusion machine suitable for producing widepanels.

FIG. 3 is a perspective view of one embodiment of a slit die of thesubject invention, showing the heating and cooling zones.

FIG. 4 is a cutaway view of one embodiment of a die from the side,showing the vertical taper in exaggerated form.

FIG. 5 is a view of one embodiment of a die from the downstreamdirection.

FIG. 6 is a further embodiment of a subject invention die from thedownstream direction.

FIG. 7 is a further embodiment of a subject invention die from thedownstream direction.

FIG. 8 illustrates one embodiment of a ram suitable for use in thepresent invention.

FIG. 9 illustrates a further embodiment of a slit die and window framerestraint suitable for use in the present invention.

FIG. 10 illustrates an embodiment of a back pressure device of thesubject invention;

FIGS. 11 a, 11 b, and 11 c illustrate further embodiments of a backpressure device of the subject invention; and

FIG. 12 illustrates a double band press as one embodiment of a verticalrestraining device and back pressure device of the subject invention.

FIGS. 13 a and 13 b illustrates a pressure adjustable side guide.

FIG. 14 a-14 c illustrate three variations of the shot marks which mightbe observed in the UHMWPE panels of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The ram extrusion process employed by the subject invention upstreamfrom the die is substantially that described in U.S. Pat. Nos.3,887,319, 3,883,631, and 4,145,175, herein incorporated by reference.

The present invention pertains to the ram extrusion of panels of highwidth and aspect ratio, as described hereinafter. The term “panel”refers to a substantially flat extrusion. Typically, the sharp cornersof the nominally rectangular cross-section may be rounded, and indeed,the entire edge portions may be circular, elliptical, etc. A purelyrectangular cross-section with sharp corners is generally not desirabledue to stress points in the die at such corners. The principle utilityof the subject invention is to produce large width sheets of constantthickness, this constant thickness extending to a position at orproximate the edge. Thus, by the definition of “panel” as used herein ismeant a generally flat sheet of less than ±0.050 inch (±1.27 mm)thickness variation over a central portion of the panel which includesat least about 80% of the overall panel width, preferably at least 90%of the panel width, and most preferably 95% or more of the overall panelwidth. Thus, it would not depart from the spirit of the invention toproduce a sheet having a “dumbbell” or other cross-section, with edgeportions significantly thicker or thinner than the majority of thepanel. These portions can easily be removed, preferably by themanufacturer but also by the end user, by conventional sawing or slicingsteps.

Because of its large width and of its asymmetry due to the high width tothickness aspect ratio, the production of wide panels from asubstantially rectangular die, i.e. a die with substantially parallelsurfaces across its width, is very problematic, and many attempts to doso have failed. Unlike many smaller products which are ram extruded atexit temperatures above the crystalline melt temperature, panels oflarge width, upon initiation of crystallization, will distort inunpredictable ways due to the volume contraction which occurs duringcrystallization as well as internal stresses which are created. However,allowing full solidification prior to exit from the die is generally notpracticed due to loss of contact with the die walls. If the die wallsare progressively narrowed toward the exit, contact can be maintained,but the buildup of pressure can at times be so high as to separate thedie halves, or even to rupture the die or its support.

The UHMWPE panels of the present invention are partially crystalline asa result of exiting the slit die of the ram extrusion apparatus at atemperature below the crystalline melt temperature. While for thin gaugepanels, i.e., those having thicknesses of 1.5-2 cm or less, the entirepanel temperature (e.g., throughout its thickness) may be less than thecrystalline melt temperature, for thicker panels especially, theinterior of the panel may be at a higher temperature. Thus, it is notnecessary that the entire panel be at such a low temperature, providedthat the surface of the panel, and preferably the panel interior to adepth of about 3 mm, is below the crystalline melt temperature. Tofurther reduce the interior temperature of thick panels, the rate ofextrusion can be slowed, thus spending more time in the cooling zone;the cooling zone may be extended; or the temperature of the coolingfluid can be lowered. These methods may be employed singly or in anycombination.

Applicants have surprisingly discovered a means of providing asolidified product without loss of die contact, while minimizing anypressure excursions. The process employs a back pressure device.However, despite the use of a back pressure device, Applicants havefound that it is very difficult to keep the process running in a stablemanner, i.e. defined as producing a flat, commercially acceptableproduct with a small reject rate.

Applicants believe that the non-robust nature of such processes is inpart due to the same factors which destroy product uniformity ofamorphous panels during cooling and crystallization, but in this casethe irregularities are exhibited within the die as opposed to outsidethe die. Applicants have surprisingly discovered that a more robustprocess can be provided if a plurality of cooling zones, preferablyminimally three cooling zones, are located proximate the exit face ofthe die distributed along the width direction, in both the top andbottom die halves, or in a monolithic die, above and below the die slit.The plurality of cooling zones are preferably arranged symmetricallyabout a centerline of the die on either the top half, the bottom half,or both. Most preferably, 5-10 cooling zones or more are utilized onboth the top and bottom, greater numbers being advantageous withincreasing panel width. Thus far, for sheets of approximately 1 m to 1.2in width, 5 to 7 zones of substantially equal size, as shown in FIGS. 2and 3 are preferably used. The top and bottom die halves need not havethe same number of cooling zones, but this is preferred.

A plurality of cooling zones are independently controlled. Each coolingzone is preferably independently controlled, although in a giveninstallation it may be found that two or more zones may be controlledtogether. Experience has shown, however, that independent control ishighly desirable and in most cases necessary. Each cooling zonepreferably has its own inlet and outlet, although outlets may be sharedin parallel with adjacent zones in suitable designs. Series connectionof all zones will not work. The zones are fed cooling fluid, which maybe gas or liquid, but is highly preferred to be liquid, due to the highheat removal demands. Cooling liquid may be water, although glycols orother heat transfer fluids may be used. Water is preferable, and aclosed system with cooling means which assures uniform water temperatureis desirable. The temperature in a given zone is measured by a suitablemeasuring means, for example by temperature sensors which are preferablythermocouples positioned in the die proximate the respective zone, andthe flow and or temperature of coolant is regulated by conventionalprocess controls. Preferably, proportional valving rather than on/offvalving is employed, and temperature regulation is preferably adjustedby varying the rate of coolant flow rather than adjusting the coolanttemperature. Temperature may also be adjusted by providing sufficientcooling to provide a lower than desired temperature, and then elevatingthe temperature to the desired temperature by electrical heating. Thisis not preferred. Temperature in each cooling zone is preferablyregulated between ±3° F. (1.7° C.) of the set point, more preferably ±1°F. (0.6° C.).

The temperatures of the individual zones are adjusted to provide a flatproduct, although as indicated previously, it may be possible to linkone or more individual zones together in a “group”. In one machineembodiment, the cooling zones positioned across one of the die halveswill be at the same temperature for the majority of the machineoperation, and the cooling zones in the lower die half will be at auniform temperature relative to each other, but at a temperaturedifferent from the temperature of the upper die half cooling zones.Depending in part upon the length of the cooling zone in the machinedirection, the temperature of the individual zones may range from 80° F.(27° C.) to 200° F. (93° C.), preferably from 90° F. (32° C.) to 180° F.(82° C.). The bottom cooling zones are in general maintained at a lowertemperature than those on the top, i.e. different by about 1° F. to 25°F. (0.6° C. to 14° C.), in general about 3° F. to 15° F. (1.7° C. to8.3° C.). It has been surprisingly discovered that the lowertemperatures produce sheets with shot marks of relatively low height.

The machine operator, observing the panels exiting the machine, willadjust the cooling zone temperature individually if warpage or surfaceimperfections are noted. For example, if the panel warps or curls in anupward direction on the far right side, the top and bottom zonescorresponding to the position of the warpage will be adjusted by makingthe top zone colder or the bottom zone warmer, or both. In general, itis desirable to have the temperature of a given zone as close aspossible to adjoining zones to minimize uneven cooling. In like fashion,if the entire sheet bows uniformly, i.e. “curls”, then the temperaturedifferential between the top and bottom zones is adjusted to remove thebow. Only minimal training of the operator is required to maintainstable operation.

The process can also be easily implemented as an automatic process inhardware and/or software, for example, by scanning the paneltransversely at one or more positions along its length by means of anoptical comparator or the like, or feel gauges, etc., which detectwarpage or bowing, and make corrections automatically. It should benoted that due to the nature of the problems associated with ramextrusion of high aspect ratio panels, numerous factors will causeinstability, for example, fluctuations in coolant temperature, rawmaterial batch changes, etc, and even the opening of a door to theoperating environment. Further information regarding the cooling zoneswill be presented in the Examples.

While the plurality of cooling zones transverse to the machine directionproximate the exit face of the die offer a great improvement inoperation, greater stability of the process is desirable. The use of aback pressure device to aid in maintaining contact has been noted.Applicants have found that the combination of multiple transversecooling zones; a back pressure device, preferably one with a pluralityof individually adjustable back pressure zones transverse to the machinedirection (as more fully described hereafter); and a dual taper slit die(as also more fully described hereafter), can produce a high quality,flat sheet material over extended product campaigns.

The panels of the subject invention should be flat, i.e. free from warpand bowing, and also of uniform thickness, preferably ±0.030 inch (0.76mm). The cross-section is thus substantially rectangular. However, thesides of the panels are generally rounded at least at the corners, sincesuch rounding lowers the pressure in the die and also eliminates sharpdie corners, which would then be points of stress which could crack atthe high pressures involved. By “substantially rectangular” is meant agenerally flat sheet when viewed from the end, with perpendicular,curved, or radiused sides, and parallel or substantially parallel mainsurfaces. For thick panels, a cross-section will actually appearrectangular, with radiused or “softened” corners. For thin panels, theradiused corners dominate the edge and the panel cross-section appearsless rectangular. However, such flat panels are still considered“substantially rectangular.” Likewise, a rectangular die slit is onewhich has two faces having a length equal to the width of the panel tobe produced, whose faces are flat and parallel to each other underextrusion pressure, the faces connected by sides which are preferablyrounded or radiused. It is the flatness of the panels, and of the diefaces which produce a substantially flat panel, which satisfy thedefinition of “substantially rectangular” herein.

It has been found, however, that if a die having substantiallyrectangular geometry when not under operating pressure is employed as adie in the extrusion of large width UHMWPE panels, the resulting panelswill not have uniform thickness, will have numerous surfaceirregularities, and in general, will be a commercially inferior andgenerally unacceptable product. For most applications, such productshave to be surface machined or skived to produce a flat surface. Thisprocess is wasteful of expensive polymer and adds further process steps,which further increases cost. For thin panels especially, the amount ofwaste would be excessively high.

It has been found advantageous to utilize a die which is manufacturedwith a slit shape which is other than substantially rectangular, inparticular a shape in which the edge regions of the top and bottomsurfaces of the die slit are not flat and parallel to each other, inconjunction with a support structure which, like the die, distorts understress, the die and support structure coacting such that distortion ofthe non-rectangular die under operating pressure transforms the dieopening into an opening such that a flat product is produced, in otherwords, a “substantially rectangular” geometry. In a further embodiment,the die and support structure are merged into one, by employing massivedie top and bottom halves. Such a massive die is not preferred, due toits cost.

By the term “slit die” is meant a die having a passage therein throughwhich UHMWPE will be ram extruded, the slit having a width considerablygreater than its height over relevant portions thereof. While numerousterms and descriptions can be used for the die and portions thereof, oneskilled in the art recognizes the basic construction of the die from thedescription herein and the drawing figures. For example, when discussingthe geometry of the slit relative to the geometry of the ram extrudedpanel, it is the shape of the product-defining portions of the die, i.e.the exit slit on the exit face of the die and a distance upstreamthereof where the panel shape is being defined which are important. Inthis context, unless specified otherwise, the term “exit slit” alsoincludes the portion of the die upstream from the panel exit slit, thisportion of the die preferably although not necessarily having the samebasic geometry as the panel exit slit per se.

For example, in a preferred embodiment, the slit will widen verticallyupstream towards the ram entrance area (“vertical taper”). Thisvertically tapered portion of the die may be considered as part of theslit of the slit die or may be considered a separate portion thereof,communicating with a generally parallel slit. Regardless, while thevertical taper may alter product properties and other factors, forexample the length of the die in the machine direction, the productshape is not defined in this section of the die or in the precedingsection, where the tongue of the ram enters and first compresses resinpowder. The various portions of the contiguous slit may, if necessary,be subdivided into a panel exit slit, i.e. the hole in the exit face ofthe die; a portion proximate the exit slit, i.e. that portion of the dieextending upstream towards the ram and terminating at the downstream endof the vertical taper; the slit taper portion; and ram inlet portion,i.e. that part of the slit beginning proximate the resin powder inlet(powder supply passage) and the point of furthest innermost penetrationof the tongue of the ram. The ram inlet portion terminates upstream withthe ram entrance slit on the inlet face of the die. “Machine direction”has its accustomed meaning, i.e. a direction parallel to the extrusionof the panel through the machine, upstream being toward the ram, anddownstream in the extrusion direction. “Transverse” means a directionperpendicular to the machine direction in a plane which includes or isparallel to a line drawn centrally across the exit slit of the slit die.

The preferred die geometry is one in which the most central portion ofthe die is of lesser height than edge regions of the die. When the dieexpands against the support structure, the edge regions of the die willdistort the least, while the central area, and the support structureabove and below the central area, will distort more, bringing the facesof the die to a substantially parallel geometry. For the purposes ofthis invention, a die will have substantially parallel geometry if,during operation, a panel of minimally 0.4 m width which has thicknessvariations over its width of less than 0.050 inch (1.27 mm) is produced.Applicants have discovered that a high quality panel of large width maybe produced by employing a die which preferably has a dual taper, andone in which the die is cooled proximate the exit such that the panelexits the die at a temperature below the crystalline melt temperature.

The dual tapers of a preferred embodiment of the subject invention diemay be divided conceptually into a vertical taper and a transversetaper. The vertical taper is a narrowing of the die proximate its inletend as viewed from the side, i.e. a narrowing in a vertical direction.The transverse taper is a narrowing of the height of the die slit in adirection transverse to the machine direction, i.e. across the width ofthe continuously produced panel stock.

The vertical taper is more clearly shown by reference to FIG. 4. The die40 is preferably machined from stainless steel in two parts 41 and 42.The die slit 43 is of substantially constant height in the machinedirection, up to the vertical taper, although it is possible to providean additional narrow taper to the die in the hot zones of the die. Thus,instead of a single vertical taper, there may be multiple verticaltapers, or a taper of progressively changing slope, the die wallsbecoming substantially parallel prior to the die exit face 44, and priorto or proximate the cooling zone. The area 45 is the ram entrance area,and has substantially constant height to accommodate ram 46. Opening 47allows UHMWPE powder 48 to be introduced into cavity 49. The ramadvances toward the die exit face 44 and stops at point 50, thuscompacting the resin powder over a length L₂. The vertical taper startsat point 50, although it may also start later, i.e. further toward thedie exit face 44. The taper, unlike those taught by the prior art foruse in manufacturing small profiles, is quite minimal, the includedangle θ, preferably being less than 10° altogether, more preferably lessthan 5°, and most preferably between 1° and 3°. The taper may besymmetrical about the centerline 51 of the die, or may be asymmetric tothe extent that all the taper is expressed in one of the die halves.Preferably, the taper is evenly divided between the die halves, and issomewhat less than 3° in total. The taper extends over length L₁. Thislength is determined by the angle of the taper and the ratio of theheight of the entrance ram channel and the nominal height of the exitslit of the die. In a die for producing panel stock of nominal 0.375inch or about 1 cm thickness, the length of the vertical taper ispreferably about 3 inches (7.5 cm). If the length of this taper is toolong, or extends substantially into the cooler zones of the die, thewell known incompressibility of UHMWPE will tend to force the die halvesapart to an even greater extent than that due to normal internalpressure.

The vertical taper, as indicated, may constitute plane walls, wallshaving a contiguous multiplicity of plane surfaces each at a differentangle from a center plane through the die, may be a continuousnon-planar surface, for example a surface with circular or generallycurvilinear cross-section, a contiguous multiplicity of such surfaces,or any combination thereof. In the case of curved surfaces or multiplecontiguous surfaces, the included angle as defined herein is the angle aplane beginning at the start of the overall taper in the die top portionand ending at the constant geometry portion of the slit (the end of thetaper) makes with a corresponding plane in the die bottom portion.

FIG. 5 illustrates one embodiment of the exit face of a subjectinvention die. In this embodiment, the die slit becomes increasinglynarrow as one progresses toward the centerline 58 of the transversedirection. The die 55, again, is shown prepared in two pieces 56 and 57.At the edge of the die slit 58, the height T₁ is substantially the sameas the nominal thickness of the panel to be produced, but is generallyslightly greater than the panel following exit from the die. The corners5 a are preferably chamferred or radiused e.g. at 59, to relieve stressand minimize friction. At the centerline of the die, the height T₂ isless than T₁. For a panel of a width of nominally 1.2 m the differenceT₁-T₂ is approximately 0.05 mm to 0.5 mm depending upon the deflectionof the die and its support structure, more preferably around 0.3 mm. Theshape of the die surface which contacts the UHMWPE may expediently be aconic section, at least in part. Examples of such conic sections areparabolae, hyperbolae, oblate and prolate ellipses, circles, etc. Itshould be noted that unless otherwise indicated, a “curvature” includesa curve of infinite radius, i.e. a straight line.

If T is the height at the edge of the die exit slit, the height at otherportions of the die may be roughly established by Te and Tm where Te isthe height at a distance De from the cavity edge, where De is 9 inches(22.5 cm) or less, and Tm is the height of the cavity at a distance Dmfrom the cavity edge where Dm>9 inches (22.5 cm). The values of Te andTm for dies of nominally 1.2 m width is then given byTe=T ₁−0.00275 De andTm=T ₁−[0.00275 De−(0.00175 (Dm−De))]

The constants 0.00275 and 0.00175 are related to the thickness of thedie (not the cavity height), the thickness of any supporting structures,the width of the die, etc. Due to the complex nature of the process andthe intricacies of the die, etc. per se, for example the coolingpassages, die length, die transition zone, etc.; for dies of appreciablydifferent dimensions, different constants and different limiting valuesof De and Dm must be determined. Resort may be had to techniques such asfinite element analysis, for example, software for which is readilyavailable. However, it may be more expedient to machine the die by trialand error, with the material removal being conservative, such that thefinal shape is approached by successive refinements without having tobegin the machining process again.

The difference T₁-T₂ may be determined empirically by production of adie having a small and preferably inadequate value of T₁-T₂, performinga trial ram extrusion, and measuring the differences in thickness of thepanel produced in a direction transverse to the machine direction. Ifsignificant differences in thickness or surface irregularities arenoted, the die can then be machined to produce a greater height variancein the slit. It should be noted that the most advantageous shape of thedie surface 60 is not necessarily circular, but may indeed be parabolic,hyperbolic, ellipsoidal, etc., and may in fact, as later discussed, bemultiply tapered.

It has been found through a great deal of effort, that the shape of thedie slit and the value of T₂-T₁ is dependent on several factors whichtogether define the requisite geometry. By the latter term is meant adie slit shape which, in conjunction with the operating parameters ofthe machine, e.g. resin type, temperature, internal pressure, length(machine direction) of die, width (transverse) of die, preferablyproduce a panel product having a width of at least 0.4 m, with athickness variation across the width of the panel of less than 0.050inch (1.27 mm), more preferably less than 0.030 inch (0.76 mm), and mostpreferably less than 0.020 inch (0.51 mm). Since these values are totalthickness variation, the surface height fluctuations are generally ±half these values, i.e. preferably ±0.010 inch (0.25 mm) or less.

The parameters which are most important in determining the transverseslit taper are: the internal pressure; the width of the panel; and thedeflection of the support structure restraining means preferably used.As the width of the panel increases, the force exerted on the two moldhalves, at any given pressure, increases also. This pressure will tendto force the mold halves apart, and in prior machines, has literallydone so. The die halves of the preferred embodiment of the subjectinvention machines are about 11-12 cm in thickness. If these die halveswere unsupported on their exterior, they would bend so much in producinga 1 cm thick panel 1.2 m wide that they would buckle and break. It wouldbe possible, of course, to make the die halves of much thicker material.However, stainless steel is expensive, and even were the die halves 25cm thick, bending would still be severe. A preferred material for thedies is T420 stainless steel, preferably polished and coated with a wearresistant and preferably lubricious coating. Conversion coatings, andpreferably metal coatings, such as hard chrome or nickel, may be used.Advantageously, diamond-like carbon (DLC) coatings such as supplied byDiamonex Products, Allentown, Pa. as Diamonex® DLC or amorphous diamondcoatings such as are disclosed in U.S. Pat. No. 6,103,305, hereinincorporated by reference, are used. Such diamond coatings are wellknown for their low coefficients of friction. Peripheral die componentscan be made of hardened steel, for example PHS 4140 prehardened steel.

The degree of distortion of the die along the direction transverse tothe machine direction is dependent upon the width of the panel beingproduced. The greater the panel width, the greater the internal diesurface area and the greater the force exerted on the die halves.Moreover, transverse to the machine direction, the distortion may beviewed as a force moment distant from the securing devices uniting thedie halves. For example, in a two piece die secured on its flanks by aseries of bolts or similar devices, distortion will be greater thefurther from the securing devices. Thus, the die will distort somewhatmore in the middle than at the edges. Moreover, the force on thesecuring bolts will receive a much greater contribution from the centralarea of the die than from the edges of the die.

To decrease the distortion of the die, a “window frame” supportstructure is preferably used with the dies of the present invention. The“window frame” consists of massive supports above and below the dies,these supports having a high modulus. Preferably, the supports are madeof carbon steel, for example Viscount®-44 steel. The supports are shownin FIG. 2 as 21 and 22 above and below the die 20. These, together withconnecting bolts 23 which are located on both sides of the structure,form a “frame” about the die halves 20 a and 20 b, keeping them incompression. In a preferred machine, the upper 21 and 22 lower supportsare about 40 cm thick or of greater thickness, and about ¾ as long asthe die in the machine direction. They are wider than the die halves bysome 10-15 cm on each side so that massive bolts may be used to tensionall together. Despite the massiveness (ca. 3 tons of steel each) of thesupports, deflection is still observed. However, the more massive thesupport the less the deflection. A plurality of individual supportsrather than a single support may also be used.

Thus, for relatively “narrow” panels of 0.4 to 1 m, by using supports ofexceptionally heavy section, for example 1 m in thickness, or by reversebeveling the supports to counter distortion, the die slit may haveminimal transverse taper or even no taper. However, such massivesupports are generally undesirable, as they not only increase machinecost, but engender a severe weight penalty as well, thus requiring astronger and hence more massive base, i.e. floor, to support the weight.

FIG. 6 is an alternative embodiment which is particularly useful forpanels less than 1.2 m in width. In FIG. 6, where the tapers areexaggerated for clarity, the die 60 is again of two parts, 61 and 62.The slit 63 has a flat section 64 in each die half, and tapered sections65, which taper at a very low included angle θ₂. This included angle isgenerally much less than 0.5°, and is preferably around 0.1°. A taper of0.05° in each mold half, for example, may be satisfactory. The junction66 between flat section and taper may be smoothed or radiused. Thetaper, transition between tapered portion and flat portion, and lengthof flat portion, are adjusted such that a flat panel is produced.

FIG. 7 illustrates a preferred embodiment of the present invention,which combines the features of both FIG. 5 and FIG. 6. In FIG. 7, thedie 70 for a panel of approximately 1.2 m width and 1 cm thickness, hasside tapers 71 as in FIG. 6, preferably extending inwards from the edgeof the die for about 10-40 cm, preferably 20-30 cm, at an angle θ₂ ofpreferably between 0.02° and 0.1°, most preferably about 0.06°. Thecentral portion 72 of the die faces, rather than being straight, iscurved inwardly (“convex”), towards the horizontal centerline of thedie, with a long radius L, for example of 200 m to 1000 m, preferably250 m to 500 m. The central portion of the die is convex on each face,and has a radius preferably of 1000/2 W where W is the width of the exitslit in meters. Coupled with a window frame support of a thickness ofabout 40 cm, it has been surprisingly found that these relativelynon-planar die surfaces, with a dual taper (side taper plus centralcurvature), do not result in a product of tapered thickness, but ratherproduce a flat panel product of highly uniform thickness and excellentsurface quality.

In a further embodiment, the slit die is machined to have a straight,non-tapered main section, or even one with a reverse taper, i.e. withgreater height in the middle than at the edges, but is used withcontoured window frame supports, as shown in FIG. 9, where again thecontours are exaggerated for clarity. Supports, 91, 92 have inwardlytapered surfaces 93, 94 which, when assembled bearing on die halves 95,96, will distort slit 97 such that the shape is not substantiallyrectangular, but is more similar to the shapes of FIGS. 5 and 6. Thus,when assembled, the die will not be substantially rectangular. Underoperation, however, the pressure in the mold will attempt to force thedie halves apart, the support structure surface abutting the die willbend upwards centered about the middle of the support, and the slit willbow outwards and distort to near net shape, i.e. to a shape whichproduces a panel product of substantially rectangular cross-section.

Thus, with respect to the die geometry, the shape of the slit is such soas to provide substantially planar top and bottom surfaces of theproduct panel when under load, i.e. at the high operating pressure ofthe ram extrusion process. This achievement is accomplished by providinga contoured mold which distorts to the desired shape under load; anon-contoured mold with a contoured and distortable window frame, or anon-contoured mold with a massive and substantially non-distortablewindow frame. For a die of ca. 1.2 m width and ca. 1 cm exit slitheight, the supports above and below the die will have to be about 0.75m thick for the latter embodiment. Preferred dies also have the verticaltaper previously described.

The rams used in producing the panels of the subject invention arestepped rams, as illustrated in FIG. 8, preferably made of high strengthsteel such as Viscount®-44 steel. The end 71 of the ram 70 whichpenetrates into cavity 45 of FIG. 4 has a thickness of just slightlyless than the thickness of the cavity, and a length approximately L₂(see, FIG. 4). The ram is preferably hydraulically operated by one ormore hydraulic cylinders, and enters the die through a seal, for exampleof polyamide. The foregoing attributes of the ram are typical of ramsused in ram extrusion of conventional shapes. From the narrow end 71toward the hydraulics (at the right, not shown), the thickness of theram increases significantly to a much thicker section 72. This thickersection reduces the flexure and any tendency to buckle which wouldotherwise occur. The thickness of the ram portion 72 may be 7 cm, forexample, while the “tongue” 71 may be 1 cm. Note that the tongue of theram, in the case of the preferred embodiment, is only slightly thickerthan the die thickness at the exit end, by a factor corresponding to thevertical taper of the die. It is also possible to employ rams having atongue of the same thickness as the die exit, but this is not preferred,except perhaps in panels of greater thickness, e.g. 4 cm, preferably 10cm or more.

The die contains at least one heated and one cooled section relative tothe machine direction. One heated section is proximate the portion ofthe die where the ram enters the die, although it may also beadvantageous to establish a cooling zone between the ram entry point orcavity 45 of FIG. 4 and the heating zone, to minimize the sintering ofresin powder near the powder inlet. The heating zone preferably beginsabout 6 inches (15 cm) prior to the beginning of the vertical taper inthe upstream direction and continues for a length downstream along themachine direction such that full consolidation of UHMWPE occurs. Thislength will vary inversely with the temperature established in theheated zone, the “hot zone”, and directly with the thickness of thepanel. Neither of these inverse and direct variables are necessarilylinear. Thicker panels require a longer period at a given temperature toconsolidate the interior of the panel. The temperature in the hot zoneis preferably from 275° F. (135° C.) to 500° F. (260° C.), morepreferably 400-450° F. (204-232° C.), most preferably about 420° F.(216° C.). Temperatures higher than 500° F. (260° C.) will tend to causeoxidative degradation of the resin, and are not desired. The hot zonemay be divided into several subzones of varying temperatures.Temperatures are sensed by conventional devices, preferably bythermocouples interfacing with automatic temperature control devices.Heating is most preferably accomplished by cartridge heaters insertedtransversely into the die.

A plurality of such heating zones is shown in the perspective drawing ofa slit die in FIG. 3. Both die halves 30 and 31 have a plurality ofpassages 32 bored through the die transverse to the machine direction.These passages may contain cartridge-type resistance heaters, or may usehot oil, etc., as a heating fluid.

The cooling section is preferably located proximate the exit of the die,and consists of a plurality of cooling passages 33, each having an inletand an outlet. Other arrangements of cooling zones, for example aplurality of inlets bored parallel to the machine direction andcommunicating with one or more common outlets bored transversely to themachine direction are also possible. The arrangement itself is notcritical. However, whatever the arrangement, the cooling zones mustallow for temperature variation to be established as desired transverseto the die slit, for example to maintain the central area of the slit ata higher or lower temperature than the edge regions, or intermediatezones.

It is highly beneficial to the operation of the present ram extrusionprocess to minimize the build-up of pressure within the die. Goodtemperature control is a necessary requirement for producing aconsistent product, but increasing temperature does not have a greateffect in lowering operating pressure since UHMWPE is gel-like andrubbery, rather than a viscous fluid. It has been discovered thataddition of small amounts of long chain fatty acids and fatty acid saltssuch as stearic acid, palmitic acid, linolenic acid, and their salts,particularly their calcium and zinc salts, aid in lowering internalpressure, presumably by lowering friction of the gel particles with thedie walls. Other lubricants such as montan esters, amides, polyethylenewaxes, silicone oils, and the like are also suitable. Such lubricantsare generally used in amounts of from 0.1 to 3% by weight, morepreferably 0.2 to 2% by weight, most preferably 0.5 to 1.5% by weight.It is important, however, to minimize the amount of lubricant to avoidloss of mechanical properties. Thus it is preferable that 1.5% or lessbe employed.

It has also been surprisingly discovered that minimizing oxygen contentof the resin powder during the process greatly lowers operatingpressure. Typically in ram extrusion, resin powder is pneumaticallydirected to a supply hopper above the ram extrusion machine by air. Ithas been found that if an oxygen-free gas is injected into the hopper,preferably at or near the bottom, to sweep or “sparge” air out of thehopper, the pressure in the die is lowered by 150 to 200 lb/in² (1MPa-1.4 MPa), a very surprising amount. This reduction in pressure mayresult in lowering the force exerted against the die halves by as muchas 1.5×10⁴ lbs pressure (67 kN) in a 1 m×0.5 m die. Any oxygen-free oroxygen-depleted gas may be used, including helium, argon, carbondioxide, nitrogen, or the like.

The UHMWPEs employed in the process are conventional powdery resins, assupplied by Ticona, Braskem, and DSM, for example. Preferred numberaverage molecular weight ranges are between 2·10⁶ Da and 10·10⁶ Da, morepreferably between 4·10⁶ Da and 8·10⁶ Da. Although the preferred UHMWPEused in the present invention is homopolymeric or substantiallyhomopolymeric, i.e. with only a low content of copolymerizablecomonomers, the compositions may, as also indicated elsewhere herein,include conventional processing aids or property-modifying additives.Examples of the latter groups of components include, but are not limitedto, components such as thermal stabilizers; UV stabilizers; nucleatingand clarifying agents; dyes and pigments; electrically conductingmaterials such as metal powders, carbon powders, graphite and the like;fillers, for example non-reinforcing fillers having BET surface areas<50 m²/g and reinforcing fillers such as fumed silica having surfaceareas ≧50 m²/g; lamellar reinforcements such as mica, expanded shale,and the like; fibrous reinforcement in the form of chopped or otherwiseattenuated fibers, for example glass fibers, ceramic fibers, aramidfibers, metal fibers, carbon fibers, thermoset and thermoplastic fiberssuch as those of polyimide, polyetherimide, polysulfone,polyethersulfone, polyether ketone (PEK) and its variants (PEKK, PEEK,etc.); tougheners such as particulate elastomers; other thermoplastics,such as non-UHMW PE; plasticizers, etc. The amounts of such additives isgenerally small.

Having described in detail individual components of the presentinvention, a preferred embodiment is a ram extrusion process employing aslit die having a width greater than 0.4 m and preferably having anaspect ratio of width to height greater than 10:1, more preferablygreater than 20:1, and yet more preferably greater than 40:1, the diehaving a top portion and a bottom portion, each of the top and bottomportions containing a plurality of three and preferably at least fivecooling zones positioned across the width of the die, the temperature ofa plurality of zones in each set of zones being independentlyadjustable. The length of the cooling zone in the machine direction andthe temperature of the zones are such that a UHMWPE panel exiting thedie has cooled below its crystalline melt temperature at its surface andis at least partially crystalline.

In another preferred embodiment, a two piece die, the exit of which isnot the shape of the desired cross-section of a ram extruded productwhen not under pressure, is contained within support structure of highstrength and modulus which bears against the die halves and restrainstheir distortion outwards at the operating pressure of the ram extrusionprocess. However, under pressure, distortion which cannot be completelyrestrained by the support structure distorts the die and supportstructure in such a way that the die exit assumes a shape which allowsthe ram extrusion of panels of at least 0.6 m width and preferably of atleast the aspect ratios previously described, the panel havingvariations in thickness of less than 0.050 inch (1.27 mm), preferablyless than 0.030 inch (0.76 mm), and more preferably less than 0.015 inch(0.38 mm). The die has at least one cooling zone in each die half,proximate the die exit, such that the UHMWPE panel exiting the die is ata temperature below the crystalline melt temperature, preferably below137° C., more preferably below 250° F. (121° C.), more preferably below120° C., yet more preferably below 110° C., still more preferably in therange of 40 to 100° C., and most preferably between 75° C. and 85° C.

In a yet more preferred embodiment, the die of the foregoing embodimenthas a small vertical taper or “narrowing” from the height of the diewhere the ram enters, to a somewhat uniform and narrower height alongthe machine direction towards the die exit. Also, in this preferredembodiment, the shape of the die transverse to the machine direction issuch that the height of the die slit is less at the center of the slitthan at the edges of the slit, and the support structure consists ofheavy steel slabs machined to have substantially flat faces which bearagainst the respective die halves. The steel slabs are tensioned by aplurality of threaded connectors, most preferably by multi-jackbolttensioners such as those available from Superbolt, Inc. Carnegie, Pa.,and described in U.S. Pat. Nos. 6,263,764; 6,112,396; 6,381,827;6,199,453; 5,083,889; and 4,846,614.

In a yet more preferred embodiment, the die of the foregoing embodimentis employed, and is also provided with a plurality of at least threecooling zones in each die half, as previously described. Thetemperatures of the various cooling zones are set to provide stableextrusion of the panel product, and are individually adjusted whennecessary to counter process variations which produce warp, bow, surfaceirregularities, and thickness variations, these adjustments preferablybeing made during operation of the machine. The product variables mostcommonly adjusted are panel flatness, thickness uniformity, and surfacequality.

In order to take advantage of the homogeneity in crystallization and thegreater uniformity of product characteristics engendered thereby, it isnecessary to maintain contact between the panel and the walls of the dieprior to the panel's exit therefrom. The die is relatively massive andpossesses a great deal of “thermal inertia,” i.e., resistance to changesin temperature. In addition, the die is temperature regulated, both inthe heating and cooling zones. By maintaining contact with the die, thethermal inertia is taken advantage of, maintaining a uniform temperatureand a uniform rate of temperature change in the UHMWPE panel. The resultis substantial uniformity in the degree of crystallization.

However, numerous factors may encourage loss of contact of the panelwith the die. One principle cause of this loss of contact is thedifferential flow rates experienced at the edges of the die as comparedto the central portion of the die. Additional fluctuations may be causedby changes in resin powder composition, the temperature of the dieproximate the exit, particularly uncontrolled temperatures across thewidth of the die, temperature excursions due to fluctuations in coolanttemperature and/or flow, material shrinkage, etc.

When the panel loses local contact with the die, the rate ofcrystallization at these localities changes, inducing changes involumetric shrinkage and also inducing stresses which accompany thesechanges. As a result, the panel may warp; bow; exhibit edge curl; or may“walk,” circumscribing a curved path, rather than a path parallel to themachine direction (a “linear path” as used herein) as it exits from thedie. Loss of contact also generates differences in local thickness, tothe extent that flatness requirements (±0.050 inch (1.27 mm), preferably±0.030 inch (0.76 mm), more preferably ±0.020 inch (0.51 mm)) cannot bemet. Since these changes often occur during established runs, theircorrection is needed if a stable process, which has a low scrap rate, isto be provided.

It has now been surprisingly discovered that contact of the panel withthe die walls can be markedly improved by imposing an adjustable backpressure onto the panel which has exited the die. The back pressure isadjustable not only in absolute amount, which is generally the leastpressure which ensures stable operation, but is also differentiallyadjustable over the width of the panel, i.e., transverse to the machinedirection.

The back pressure is provided by a back pressure device locatedproximate to the die exit. It should be noted that the back pressuredevice causes no or substantially no change in the thickness and/or thewidth of the panel, because the panel is a solid at this point, beingbelow the crystalline melt temperature. Rather, the pressure exerted isa frictional restraint on movement of the panel away from the die, andwhich translates to the die exit and even into the die interior, e.g.,to the heated portions of the die, forcing the UHMWPE to maintaincontact with the die walls substantially throughout the length of thedie, but most importantly over the cooled section proximate the die exitslit.

The location of the back pressure device is preferably as close to thedie exit as is conveniently possible. A distance of 1 cm to 20 cm,preferably 2 cm to 10 cm is preferable, for example. Greater distancemay allow for buckling or bowing of the panel due to the back pressuregenerated, particularly with thin panels of high aspect ratio. Suchpanel distortion may not be easily recoverable, and thus close spacingbetween the backpressure device and the die is preferred.

The position of the plurality of back pressure generating elementsacross the width direction of the panel is desirably symmetricaltransverse to a centerline of the panel in the machine direction, butsmall departures from symmetry can be tolerated. For dies which havebeen used for extended periods and which exhibit non-uniform wear in thedie interior, purposeful deviation from symmetry may be desired.However, assuming a symmetrical and uniform die contour, a symmetricalarrangement of pressure generating elements is preferred. Working intandem with the back pressure device are optional pressure adjustableside guides that help contain and center the extrusion under the backpressure fixture. These work as a guide to hold in position an extrusionthat might otherwise naturally tend to move to the left or right underdifferent back pressure settings across the width of the extruded panel.

The back pressure device may take numerous forms, so long as the deviceoperates to impede the movement of the UHMWPE panel in the machinedirection, thus imparting back pressure, and preferably isdifferentially adjustable across the width of the panel such thatportions of the panel may experience greater or lesser back pressurethan others. For example, the device may consist of a row of textured orelastomer-coated upper and lower roll pairs distributed across the widthof the panel, which press against the panel and are frictionallyrestrained in rotational movement, this rotational restraint beingadjustable between roll pairs the rotational restraint may be achievedthrough the use of electromagnetic braking devices, hydraulic devices,etc. The textured surfaces of the rollers, when used, should be such asto avoid leaving an impression on the smooth UHMWPE panel surface. Mostpreferably, however, the back pressure device consists of one or aplurality of “boards” spaced across the width of the panel, and aplurality of pressure applying means, for example adjustable bolts,adjustable spring devices, or preferably, hydraulic or pneumaticcylinders, all of which may be termed “pressure actuators.” While the“boards” may be made of numerous materials, such as fiberglass or carbonfiber composite materials, thermoplastics, metal, etc., it is highlypreferred that the board(s) be a wood product, preferably solid woodlumber or plywood, preferably covered with a non-marring woven ornon-woven textile material.

Other back pressure devices include any devices which can retardmovement of the extruded panel away from the die such that a backpressure is created. Since the UHMWPE panel is extruded below the melttemperature, the panel may be gripped by clamps along the edge, may berestrained by servo-driven continuous belts, etc. For example, aplurality of such belts arranged parallel to each other in the machinedirection may be used. Pressure may be supplied as well be inflatablebladders, including metallic bladders or bellows. These may beappropriately coated to provide the desired amount of sliding frictionin order that the panel may advance away from the machine while stillbeing restrained.

The vertical pressure on the back pressure elements is preferablysupplied by hydraulic or pneumatic cylinders, preferably the latter. Forexample, 4 inch (100 mm) air cylinders, one or more bearing against eachpressure element, and operated at up to 100 psig (6.9 bar gauge),preferably 25 (1.7 bar gauge) to 100 psig, have been found satisfactoryduring extrusion of a ca. 1 cm thick by 1.2 m wide panel. Greaterflexibility in operation can be achieved if each side of the pressureelements, when individual pressure elements are used, are independentlysubjected to pressure. Thus, for example, each individual upper pressureelement, which may be slightly in excess of 1 m in length and 0.4 m inwidth, may be actuated by right hand and left hand pairs of pneumaticcylinders. For added pressure, these pairs of pneumatic cylinders may bedoubled or tripled, for example with one pair at the upstream end of theboard and one pair at the downstream end.

One example of a back pressure device is shown in FIG. 10 inperspective. The die is not shown for clarity. The UHMWPE sheet 111 hasexited the die (not shown, to the right) at a temperature below thecrystalline melt temperature. The back pressure device 112 consists oftwo platens in the form of sheets 113, 114 of wood or plywood, the upperplaten 113 for example, of 0.5 inch (12.5 mm) thickness. The lowerplywood sheet, which is preferably of 1.0 inch (2.5 cm) thickness or ofgreater thickness, is maintained in position by fastening to a supportstructure 115 supported by legs 116. The support structure may be anyconvenient structure having the necessary rigidity, for example, astructure made of aluminum or steel channel, etc. Following the backpressure device in a preferred embodiment, are a series of freelyrotating roll pairs 117 a, 117 b which may be made of any suitablematerial, for example, mild steel, stainless steel, or aluminum, andwhich are suitably positioned above and below the UHMWPE panel, forexample, by a pair of side rails. One side rail 118 is shown, the frontrail being deleted for purposes of clarity. The ends of the rolls mayhave pins located in sockets in the support rails, may be constrainedwithin ball bearing races, etc. The roll pairs may be located such thatthey lightly bear against the panel; may, particularly if solid, rest onthe UHMWPE panel, or may be adjusted in position to just contact or toavoid contact by a narrow space, for example, but not by way oflimitation, 0.010 inch (0.25 mm) or 0.005 inch (1.25 mm). The purpose ofthe roll pairs is to assist in maintaining panel flatness as the panelcools further, not to alter back pressure, although if they bear againstthe panel and are restrained in movement, they may assist in thisfunction as well. The rolls are advantageously 1.0 inch to 4.0 inches(2.5 to 10.2 cm) in diameter, more preferably 1.5 inch to 4 inches (3.8to 10.2 cm) in diameter, and are preferably relatively closely spaced inthe machine direction; e.g., at a spacing of 0.10 to 3.0 inches (2.5 mmto 7.5 cm) (circumference to circumference), preferably 0.2 (5 mm) to2.5 inch (6.4 cm) apart. As indicated previously, the roll pairs areoptional, and can in fact be used with back pressure devices other thanthose having a plurality of pressure elements, and can be used alone aswell.

Other methods of vertically restraining movement of the panel and/orportions thereof may be used instead of a plurality of roll pairs. Forexample, a double belt press may bear lightly against the panels and beconfigured such that rotation is in sync with the incremental extrusionof the panel, or may be driven by panel movement itself. Likewise, oneor more panels such as those used as back pressure elements, preferablycloth or felt covered, or made of or covered with lubricious material,may be fixed permanently in position just touching the panel, oradjustable to such a position, such that minimal restraint is exertedagainst the panels, while vertical movement of the panel issubstantially prevented. The panels, bars, or other devices may bepositioned parallel the machine direction or transverse thereto. Othermeans of accomplishing the same result can now readily suggestthemselves to the skilled artisan.

Arrows 119, 120, and 121 illustrate points at which pressure is appliedto the back pressure device, preferably by hydraulic or pneumaticcylinders positioned above the back pressure platen 113. While threepoints of pressure are shown, two pressure points, for example 119 and121 may be used, or a higher plurality, i.e., four, five or six,generally less than ten, pressure points may be used. Pressure may alsobe applied from the bottom, i.e., to bottom platen 114, or to bothplatens 113 and 114.

UHMWPE when heated above 180° F. (82° C.) begins to lose strength, andbecome less rigid. Due to the extreme width to thickness ratio of widepanels of the material, when exiting the die above 200° F. (93° C.) thematerial will move at different rates along the width. This causestearing, and humping. Several factors contribute to these observations,including die cleanliness at start up and which remains in a state offlux, pinch point condition, die plate flex, die plate condition, andhang up. Multiple post extrusion pressure zones allow different sectionsof the sheet product to be adjusted according to the observed condition.Areas wanting to hump may have additional pressure applied to them,while areas that tear will have their pressure reduced. Surface qualitymay thus be improved.

Bow, both horizontal and vertical, is caused due to stress and unevenheating and or cooling of the material. Post extrusion pressure zonesmay be used to counter uneven slip and drag in the tool that causestress. The multiple zone configuration allows adjustments to be made inlocalized areas of the sheet, depending on the direction and type ofbow. Basically, the pressure zones are used to even the flow ofmaterial, across the width, as it exits the tool.

In operation, the operator observes the flatness or other productcharacteristic of the panel as it travels between the roll pairs, orafter exiting the roll pairs, for example prior to or after cutting tolength. In one embodiment, side position sensors 122, 123 may beinstalled to detect sidewards motion. If the sheet begins to exertpressure on sensor 122, for example, this is a sign that the opposingside may be moving through the tool and exiting at a higher output rate,or may have inadequate contact with the die and is cooling too slowlyand shrinking differentially. Accordingly, greater pressure would beexerted on pressure point 119, or lesser pressure on pressure points120, 121. Likewise, thickness measurements indicating a thinner panel onthe left side (left when facing the die exit) imply slower cooling,greater crystallinity, and correspondingly greater shrinkage hasoccurred as compared to other points. This may indicate inadequatecontact of the right side of the panel with the die, allowing slowercrystallization due to loss of contact with the cool die proximate theexit. Increasing back pressure on the right side by applying greaterpressure at pressure point 119 is warranted. In summary, the verticalpressures on various zones of the back pressure device transverse to themachine direction are adjusted to maximize flatness and dimensionalstability. This cannot ordinarily be accomplished by a single backpressure zone; a plurality of zones is required, although once stableoperation is attained, it is not unusual that little, if any furtherchange, be made to the back pressure device, even for long runs.However, the pressure settings may vary from run to run, and will varymost between start up and stable operation.

In like manner, if the thickness specification is ±0.020 inch (0.51 mm),the panels have not exceeded ±0.010 inch (0.25 mm), but suddenly orgradually tend towards greater variations in thickness, for example oneside or a central portion increasing in thickness such that continuedincrease over time will exceed the ±0.020 inch (0.51 mm) thicknessspecification, then the back pressure device is adjusted over a portionof the width of the panel to discontinue this trend, or to reverse it.

The same device can be used to run materials other than UHMWPE, as wellas UHMWPE with differing additive packages, and may be adjustedaccordingly depending on the pressure requirements for each particularmaterial. For example, other polymers which are gel-like above theircrystalline melt temperature, such as PTFE, can be employed.

In addition to providing differentially adjustable back pressure acrossthe width of an extruded panel, the back pressure devices of the subjectinvention can also be uniformly actuated or subject to the same increaseor decrease in pressure across the width when the pressure in the die isin need of adjustment. In a slit die of large width, under ram extrusionconditions, large internal pressure are generated. The geometry of thedie takes into account the deflection which will occur at the expectedoperating pressure such that a flat panel is the extruded product. Ifthe pressure in the die is reduced, for example by a change in internaltemperature or batch-to-batch resin variation, the height of the die,particularly at its mid-section, will decrease. The result will be apanel which is thinner in the middle than at the edges. Likewise, anincrease in pressure will cause the die to distort more than planned,and the panel product will now be thicker at the middle than at theedges. Increasing the pressure on the back pressure fixture elementswill increase the internal pressure in the die and vice versa, and thusmonitoring the relative thickness of the central area of the panel withthe edge thickness will alert the operator or control device as to whena global increase or decrease in back pressure is mandated to keepproduct within specification and preferably to optimum thicknessvariation. Thus, by varying the back pressure on the panel exiting thedie, the die geometry is altered by a corresponding change in internalpressure. However, such adjustments are not reliably repeatable, andthus this aspect is generally used only for relatively coarseadjustment.

Wood back pressure devices, such as those of FIG. 10 (113, 114), aregenerally coated with soft fabric, woven or non-woven, to minimizescratching of the UHMWPE panel surface. The panels may also be facedwith soft plastic, but this is less desirable, as such facings mayacquire grit embedded into the surface, causing scratching. Whilefabrics and felts may also suffer from this problem, they are easily andinexpensively changed.

A potential disadvantage of the back pressure device of FIG. 10 is thatthe back pressure across the back pressure device is not trulyindependent, since the pressure exerted, for example, at pressure point120 will also be exerted in part at points 119 and 121. The more rigidthe sheets 113 and 114, the less independent the pressure adjustment,and thus in this embodiment, the sheets which make up the back pressuredevice are preferably relatively thin, or are scored to be flexibleacross their width, i.e., flexible transverse to the machine direction,by scoring parallel to the machine direction, through the use of linkingdevices or hinges, etc.

A more preferred embodiment of a back pressure device is illustrated byFIGS. 11 a, 11 b, and 11 c. In FIG. 11, the back pressure device isformed of three upper pressure platens 131, 132, 133 of 0.75 inch (19mm) thick wood or plywood. The back edges in the machine direction areprevented from drifting downstream with the ram extruded UHMWPE panel135 by end restraint 136 which has a downwardly extending lip 137.Located above the upper pressure platens proximate their upstream endsis pressure actuator support 138. Both the end restraint 136 andpressure actuator support 138 are fastened to C-channel rails 140 byfasteners 139. Positioned above each upper pressure platen (131, 132,133) is a pressure actuator, here, respective pneumatic cylinders 141,142, 143 which are fed pressured gas by supply lines 144, 145, 146. Theshaft, or plungers of the pneumatic cylinders are attached to thepressure platens by any convenient method. As shown in FIG. 11 b, a sideview of the back pressure device and associated equipment, shaft 147 isattached to platen 131 by means of bracket 148 and pin 150. Bracket 148is attached to platen 131 by screw fasteners 149. Devices such asbladders, independently driven wheels, flexible plates deflected byservo, stepping devices or thermal expansion devices may be used inplace of pressure cylinders to actuate the back pressure device.Substantially any device which allows for variation of pressure appliedto the back pressure elements is suitable.

Mounted below the passageway for the UHMWPE panel on lower frame rails152 is the single (in this embodiment) lower platen 156 which is firmlyattached to frame rails 140 and 152, and supported by crossmembers 157to provide rigidity. The UHMWPE panel 135 passes between upper pressureplatens 131, 132, 133 and lower platen 156. The upper platens, lowerplaten, and pressure actuators, with their necessary supports andlocating devices, etc., form the back pressure device.

Following the back pressure device are vertical movement restrainingroll pairs 153 a,b, 154 a,b, 155 a,b, as shown in FIGS. 11 a, 11 b.These roll pairs are journaled into the upper and lower frame rails 140and 152, and are sized and positioned so as to just contact the UHMWPEpanel 135. While three roll pairs are illustrated, it is desirable tohave twenty or more roll pairs. The roll pairs serve a dual function ofremoving heat in a relatively uniform manner while also maintainingflatness of the panel. In this manner, the roll pairs, while preferablyemployed together with back pressure devices having independentlyadjustable pressured zones, can also be used together with a backpressure device having but a single pressure zone, although this is notpreferred. Between channels 140, 152 are spacer strips or blocks tosandwich the wood lower platen between the rails. Alternatively, thelower platens may be fastened to crossmembers 157 or otherwisesupported, e.g. by a welded-in frame, etc. The apparatus illustrated inFIG. 11 can be constructed in numerous ways and out of numerousmaterials, by one skilled in the machine and fixture construction arts.

A similar embodiment to FIG. 11 which is less preferred, employsmultiple lower “platens” below the extruded panel, thus taking the placeof single platen 156. The platens are preferably made of wood materialin all the embodiments due to its low cost and excellent thermalproperties. Wood products which exhibit minimal distortion arepreferable, for example layered wood products such as plywood. Solidwood products are desirably stable species such as mahogany. Layeredwood products having reinforcing layers or facings such as resin matrixwoven or non-woven fiberglass or carbon fiber are also useful. If madeof thermally conductive materials, it is desirable that the platens haveeither a high thermal mass (inertia) or contain insulative layers, orboth, to minimize temperature fluctuations and maximize heat retention.

For the extrusion of a single panel, the amount of downwards pressureexerted against the panel, which retards movement of the panel(“restraining force or friction”) and thus gives rise to back pressure,depends upon the thickness of the panel being extruded and its width.The pressure preferably should not cause the panel to bow or bendbetween the exit die and the back pressure device, and thus for panelsof a fixed width, the thinner the panel, the less back pressure can betolerated, and the pressure applied to the extruded panel in a verticaldirection, i.e., the “restraining pressure,” is less than for a thickerpanel of the same width. These considerations, together with thedesirability of maintaining contact of the panel with the walls of thedie at least in the cooling zone, limit the minimal thickness of theextruded sheet to approximately 3-4 mm in thickness, and preferably 5-6mm, assuming a width of 0.4 to 0.5 m.

The minimum width of the panels is about 0.4 m, more preferably 0.5 m,and yet more preferably at least 0.6 m, with widths of 1.0 m to 1.3 mpreferred. The maximum width is limited only by the ability tomanufacture a suitable die, and for practical reasons is generally lessthan 3 m, and more preferably about 1.5 m (5 ft) or less.

The aspect ratio of the panels is also important. The minimum aspectratio (width divided by thickness) for panels above 0.6 m in width ispreferably about 20, more preferably above 40, yet more preferably above60, and desirably above 80, for example 100-250. As one example, a 1.2 mwide panel with a thickness of 1 cm will have an aspect ratio of 120. Atlower aspect ratios than 20, it may be difficult to apply sufficientback pressure by the means described heretofore, without affecting thesurface finish of the panel. For smaller panels, i.e. in the range of0.4 to 0.6 m in width, an aspect ratio greater than 10 is preferable,more preferably greater than 20, yet more preferably greater than 40,and still more preferably greater than 60. Aspect ratios greater than100 are particularly preferred.

Thus, in one embodiment, the invention is directed to a back pressuredevice having differential pressure zones for use with UHMWPE panels ramextruded through a die having a substantially rectangular cross sectionunder operating pressure and having a multiplicity of transverse coolingzones proximate the exit face of the die, wherein the panel has asurface temperature below the crystalline melt temperature upon exitfrom the die. The invention is also directed to the use of a pluralityof vertically confining roll pairs or other vertical restraintsdownstream from a back pressure device, even one without a plurality ofdifferential pressure zones, which confine the panel dimensionally untilthe temperature of the panel is in the range of ambient to 85° C., morepreferably ambient to 50° C., yet more preferably ambient to 43° C. andstill more preferably, to a temperature within the range of 32 to 37° C.While the lowest preferred temperature is the ambient temperature, itwould not depart from the spirit of the invention to cool the panelbelow ambient.

It should be noted that the back pressure generating elements which aredifferentially adjustable may individually be set at zero orsubstantially zero pressure. Thus, some back pressure elements may onlylightly contact the panel, while others may exert considerable pressure.It should also be noted that the difference in pressure is not requiredto be totally independent, as also explained elsewhere. All that isrequired is that a difference in pressure between at least two andpreferably all back pressure generating elements may be established.

The side guide devices are located on both sides of the panel exitingthe die, preferably in the neighborhood of the back pressure fixture.The side guides may constitute a pin or bar positioned vertically, whosesides bear against the panel. However, to avoid any impression on theedges of the panel by the side guides, the latter are preferably in theform of a horizontal bar or channel, preferably one of wood, clothcovered wood, or lubricious plastic, for example a UHMWPE or nylon-facedmetal channel.

Reference may be had to FIGS. 13 a and 13 b which illustrate sideguides. For simplicity, the framework and other details have beenomitted. The side guides may also be linked to a side pressure measuringgauge which senses sidewards pressure of the panel, and causes pressureon the proper edge of the panel to be increased to counter the panel'sattempt to escape parallelism with the machine direction. In otherwords, the panel ideally exits the machine straight, i.e. with the edges(and hence also the center, etc.) of the panel parallel to a line in themachine direction. However, differences in cooling and/orcrystallization rates or other factors such as non-uniform pressureacross the panel exerted by back pressure elements may cause the panelto attempt to move sideways. The side guides and sensors associatedtherewith are used to counteract this tendency against parallelism,should it occur. Since this sideways panel movement is most likely tooccur at the back pressure device, it is preferably that the side guidesare located at the back pressure device location, or immediatelyfollowing the back pressure device.

The back pressure fixture of FIGS. 13 a and 13 b consist of a lowerboard or platen 151 and three upper platens 152, 153, 154. Pressure isapplied through means of the platens to UHMWPE panel 155 by pneumaticcylinders 156, 157, and 158 which bear, respectively, against platens152, 153 and 154. There are 12 pneumatic cylinders in all, two on theleft side of each platen, as shown in FIG. 13 a as 156 a, 156 b, and twoon the right side. The cylinder rods are attached to the platens bybrackets 159 and clevis pins 160.

The side guide devices 161 consist, as illustrated in FIG. 13 b, of alubricious thermoplastic bearing surface 162 which is bonded orotherwise attached to metal channel 163. Pneumatic cylinder 164 appliespressure through cylinder rod 165 which is secured to metal channel 163by bracket 166 and clevis pin 167. Other fastening means are alsosuitable, and other pressure arrangements. Two such devices are shown inFIG. 13b, one on each side of the panel 155.

The side guides are shown in FIG. 13 a from the side. In this case, twoside guides are used on each side of the panel, in order that greatersideways pressure can be applied.

The restraining of movement of the panel away from the die is mostpreferably a frictional restraint. This frictional restraint may be asliding friction where there is relative movement between the UHMWPEpanel and the surface of the back pressure devices, for example, thoseillustrated by FIGS. 10 and 11, or may be a restraint which does notallow for sliding friction. Examples of the latter are elastomer coatedrollers which rotate in a restrained manner, but grip the panel surfacein a non-sliding manner, or a double band press as described below.

An embodiment with a double band press is illustrated in FIG. 12.Traditionally, double band presses are employed to consolidate materialssuch as thermoplastic matrix, fiber-reinforced semi-finished products byapplying a consolidating pressure, and often heat as well. In thepresent invention, a rather simple form of double band press may be usedas a vertical restraining means following a back pressure device, or asillustrated by FIG. 12, may also serve as the back pressure deviceitself. In the former case, the bands or belts of the “press” bearagainst the UHMWPE panel, and are not necessarily driven, the frictionalcontact with the sheet providing the driving force for the bands and therolls supporting them. In the latter case, the bands are drivensynchronously but delayed with respect to movement of the ram of the ramextruder such that a back pressure is created between the double bandpress and the die.

In FIG. 12, the double band press 170 is constructed with two continuousbands or belts 171, 172 which bear against UHMWPE panel 173 extrudedfrom ram extrusion die 174 (also refer to FIG. 1). Located above thelower portion of the top band are platens 175, 176, 177, 178, andlocated below the upper portion of the lower band are platens 179, 180,181, 182. These platens may be adjusted to bear the bands against thepanel from an essentially pressureless contact to a relatively highpressure. Due to the incompressibility and relatively high strength ofthe panel, a quite low pressure, due to the large surface area of thebands, will prevent movement of the panel except when such movement isdesired.

The platens may also be heated or cooled. Since the travel of the panelis relatively slow, the panel can spend a considerable time within thepress. Thus, for example, the beginning of the press can be heated tothe crystalline melt temperature and maintained at this temperature orslightly above or below to anneal the panel or promote additionalcrystallization of the UHMWPE. Care must be taken to ensure temperatureuniformity in such cases.

The press in this embodiment is driven by electrical motors or othermeans (not shown) in an incremental fashion synchronously but delayedfrom the travel of the extrusion ram 184. Thus, as the panel attempts toexit the die, it will experience a considerable initial pressure betweenthe press and the die since the press at this point is stationary. Ifthe press remained stationary, the panel would greatly distort. However,the driving means is tied to the movement of the ram by feedback loop185, and the driving motors on/off cycle and speed during their oncondition are regulated by a conventional process controller or PLA 186.The start of movement of the bands may be delayed by from 0.01 to 0.10second, for example. The normal delay in beginning rotation of the pressmay provide this delay.

In a general aspect, the present invention is directed to a process ofram extrusion of UHMWPE panels having a width in excess of 0.4 m, morepreferably >0.6 m, and yet more preferably in the range of 1.0 to about1.5 m, comprising the steps of:

ram extruding an UHMWPE panel through a slit die wherein the temperatureof the panel exiting the die is below the crystalline melt temperature,the die preferably having a multiplicity of transverse cooling zoneslocated proximate the exit face of the die;

restraining movement of the panel away from the die such that a backpressure is created, a back pressure device preferably having aplurality of back pressure zones positioned transverse to the machinedirection, and adjustable relative to other back pressure zones;

optionally, guiding the panel's horizontal movement transverse to themachine direction;

monitoring at least one characteristic of the panel which has a definedproduct specification value;

determining when the at least one characteristic is outside the productspecification value or is exhibiting a change in the characteristic awayfrom a desired value of said characteristic; and

adjusting the back pressure differentials or the temperatures of thecooling zones or both along the width of the panel to bring the at leastone characteristic within the product specification or to counteract thechange in the characteristic away from a desired value of thecharacteristic. The characteristics which are most important are panelflatness, thickness uniformity, and surface quality.

It is an object of the present invention, therefore, to provide a stableprocess. By a stable process is meant a process where long runs ofproduct, runs lasting, for example for minimally 8 hours (1 operatingshift), to several days, or to weeks or even months, without producingsignificant amounts of off-spec products. In a typical 3-7 day run, forexample, a scrap rate of less than 10%, preferably less than 5% would beexpected. The stability of the process is achieved either automaticallyunder suitable process control, or by an operator. It has been foundthat operator training is rapidly accomplished.

The product produced by the inventive process is a novel product whosemanufacture was not possible prior to the present invention. Asindicated previously, due to the difficulties of ram extrusion, it hadbeen impossible to ram extrude constant thickness panels of large widthand high aspect ratio, i.e. thin panels. Thus, to produce thin panels,thick, compression molded panels were skived to produce thin sheets.However, these sheets have irregular surfaces, both in terms ofthickness variation as well as appearance.

The UHMWPE panels produced by the subject process are, in general,superior to products of similar dimensions which are produced bycompression molding of large, thick blocks followed by skiving.Noteworthy among the superior characteristics are a more uniformthickness, freedom from edge curl and end curl, and a smooth glossyfinish. The surface normally is characterized by parallel shot markswhich are difficult to observe except at oblique angles. Each productwhich is commercially acceptable must meet product specifications, whichmay constitute a minimum value of a particular characteristic, a maximumvalue, or a range of values. For example, a nominally 1.0 m wide sheetmay have a width tolerance of ±0.5 mm; or −0 mm, +0.5 mm, etc.,Likewise, thickness variations may be expressed by ranges such as ±0.050inch, ±0.020 inch, −0, +0.30 inch. etc. Edge curl may be set to adistance from the edge towards the center of the width of the sheetwhere edge curl is absent, i.e., up to 1 cm, up to 2 cm, etc. Similarly,warp, bow, and end curl may have values set for them.

The foregoing product specification values are generally set by themanufacturer, the distributor, or the end user. Regardless of whodetermines what product characteristics are necessary, the manufacturermust adjust the process in order to meet these specifications. All theseproduct characteristics are easily measured by methods well known in theart. Edge straightness, for example, can be measured by abutting theedge of a sheet against a reference flat surface and noting departuresfrom contact. Thickness may be measured by feel gauges, by profilometry,optical interferometry, etc. Many of these measurements can be madefollowing the cutting of extruded panels to size, employing astatistical sampling regime. Other measurements can be made on line onthe moving panel, especially thickness variations, and any tendency forthe panels to “walk.” Some characteristics, such as warp, bow, andwalking will be evident to the machine operator, who will then make thenecessary adjustments in back pressure and/or cooling zone temperaturesto restore the characteristic to its desired value. For example, a panelwhich is tending to walk towards the left can, following adjustment, bemade to exit the die and continue downstream in a linear fashion bysuitable adjustment of back pressure on a portion of the panel relativeto other portions. A panel which has not “walked” and thus whose sidesare at right angles to its length, is said to be “orthogonal”, and themeasurement of “walking” may be described in terms of “panelorthogonality”. The edge of a panel which has walked and is thus notorthogonal will exhibit a curved edge when compared to a straightreference surface. In very unstable processes, S-shaped curves wherewalking fluctuates between left and right directions may also occur. Theside guides described herein may also assist in preventing “walking”.

The smoothness can be assessed easily by touch, and the gloss of thesurface assessed visually, or by optical inspection techniques such asthe standard ASTM D2457-03 and D523-89 test methods, preferably at a 60°angle. Using this test methodology, employing a BYK Gardner Color GuideGlass Meter supplied by BYK Gardner U.S.A., Columbia, Md. and a 60° testangle, average gloss measurements preferably range from 35 upwards, withmost panels in the range of 35 to 45. This is in contrast to skivedpanels which, although also smooth, have significantly lower gloss,averaging about 24 using the same test methodology.

The uniform surface has a 3σ thickness variation of less than ±0.050inch (±1.25 mm), more preferably within ±0.030 inch (±0.76 mm), and yetmore preferably within ±0.020 inch (±0.50 mm). While the surface issmooth and glossy, it can further be distinguished from skived panels bythe presence of shot marks substantially perpendicular to the length ofthe sheet (in the machine direction) and substantially parallel to eachother. These shot marks are believed to be caused by the stationarydwell between ram extrusion cycles, and the profile and height of theshot marks can be measured by standard profilometry techniques. The shotmarks have a maximum height (top to bottom) of only several microns,i.e. less than 70 μm, more preferably less than 25 μm, and importantly,do not interfere with known uses of thin panel stock. Products have beenproduced over extended trials with shot marks measuring about 5-10 μm inheight. The shot marks may easily be seen by observing the panel at anoblique angle.

FIGS. 14 a-14 c illustrates several shot mark patterns. In FIG. 14 a, an“ideal” shot mark pattern is shown, where the delimiting edges 201 ofshot marks are exactly parallel to each other and perpendicular to themachine direction in panel 200. This type of pattern would result if theUHMWPE in the die all flowed at exactly the same rate, experienced thesame back pressure, etc., and these parameters did not change over thelength of the panel.

However, the flow of UHMWPE is generally retarded somewhat at the edgesof the die, and may experience less retardation at the die center. Theretardation at the two edges may not be the same either. The result isthat the shot marks are typically somewhat “wavy” or “undulating” acrossthe panel surfaces (shot marks are on both front and back surfaces) asshown at 202 in FIG. 14 b. The generally perpendicular orientation tothe machine direction is clear, as is also the generally parallelorientation with respect to adjacent shot marks. In FIG. 14 c is shown asimilar panel but one in which the shot marks change their respectiveorientation along the panel, initially at 202 being the same as in FIG.14 b, departing somewhat over portion 203, and becoming similar againover portion 204. The shot marks are still substantially perpendicularto the machine direction and substantially parallel to each other.

Thus, the present invention also pertains to an UHMWPE panel having awidth greater than 0.4 m, and characterized by generally parallel shotmarks on both top and bottom surfaces. For panels whose length directioncorresponds to the machine direction, these shot marks are across thewidth of the panel, i.e. transverse to the machine direction. Thesurface of the panels are smooth and glossy, and have a thicknesstolerance as previously described. Preferred panels are from 4 mm to 10cm in thickness, more preferably less than 2 cm thick. Preferred panelshave a global flatness variation of less than 0.150 inch (3.8 mm; 3σ)with a tolerance of ±0.030 inch (0.8 mm; 3σ), and preferably a globalflatness variation of less than 0.100 inch (2.5 mm) with a tolerance ofless than ±0.024 inch (0.6 mm; 3σ). The sideways bow (curvature) of thepanels, without trimming, is preferably within approximately the samevalues as the global flatness. It should be noted that these sidewaysbow tolerances for the panel as extruded, are superior than that whichis ordinarily obtained by conventional sawing of the edge. Thus, it ispreferred that the subject invention panels retain their edges “asproduced”, and are not sawn.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXAMPLE 1

A ram extrusion machine is provided having a stainless steel diecomposed of two halves, substantially as described herein, asillustrated, for example, by FIGS. 2, 3, and 4. The die has a length inthe machine direction of 81.25 cm, a width of 172 cm, and a height of96.5 cm. The bottom die half is similar to the top half but does notcontain any passageways 147 for entry of UHMWPE powder, and has edgesconfigured to mate with those of the top half. The die is verticallytapered in the machine direction from the ram entry by about 3°, and thegeometry of the slit is generally circular with a very long radius, ca.375 m. The hot zone of the die is maintained at 440° F.±20° F. (227°C.±11° C.), and the die exit end is provided with five cooling zones oneach die half, extending approximately 3 inches (7.6 cm) into the die inthe machine direction.

The die is contained within a support structure consisting of two flatsurfaced carbon steel billets measuring 68 in. (1.7 m) by 15 in. (0.38m) by 22.5 in. (0.58 m), which are tensioned by 5 multi-jackbolttensioners having a bolt diameter of 2 in. (5 cm). The steel billets arepositioned over the die about one inch from the exit end and extendingfrom that point toward the ram entry end of the die, as illustrated inFIG. 2.

UHMWPE resin powder 6540 from Braskem, Sao Paulo, Brazil, having a melttemperature of 133° C., an average particle size (D50) of 190 μm, and anaverage molecular weight of 8×10⁶ g/mol is introduced pneumatically intoa hopper above the UHMWPE entry ports in the top die half. A flow ofnitrogen is introduced into the hopper near the bottom to remove airfrom the resin powder. The ram is drawn back and resin powder is allowedto fill the die cavity below the hopper. The ram then enters the die andcompresses the resin powder and pushes it into the vertically taperedportion of the die, this resin in turn pushing the previous portion ofresin powder, which has now been heated to a gel, towards the die exit.The ram dwells at this position for approximately 1 second, and then thecycle is repeated. At the beginning of each cycle, once the die is full,0.5 inch (12.5 mm) of length of an UHMWPE panel of 0.375 inch (9.5 mm)thickness and width of 4 feet (1.22 m) exits the die. The panel ispartially restrained in movement by a cloth covered back pressure devicewhich facilitates continued contact of the cool (250° F.) panel with thewalls of the die. The back pressure device is constructed so as to allowfor variations in back pressure across the width of the panel. The panelis taken up on a flat conveyor, and the operator adjusts the coolingzone temperatures to remove warp and bow from the panel. The top andbottom cooling zone temperatures at stable, flat panel production, are[42° C., 56° C., 60° C., 49° C., 43° C.] and [42° C., 49° C., 53° C.,44° C., 41° C.] respectively. The panel exits the die at a temperaturebelow 100° C., much lower than the crystalline melt temperature. Theflat panels produced are cut to length, and have a glossy flat surfacewith a thickness variation of less than 0.030 inch. Mold “shot marks” ofa length equivalent to each ram stroke's production can be seen ifobserved carefully at a high angle of incidence, but are virtuallyinvisible when viewed head on. The panel has slightly rounded edgeswhich are uniform from panel to panel, and is of commercial quality.

EXAMPLE 2

The same machine and process are used as in Example 1, but the die hasin addition to the vertical taper, non-parallel exit faces having dualside tapers and a curvilinear top section, as described in FIG. 7. Thetop cooling zones are all set to 76.7° C. and the bottom zones all setto 73.9° C. Very little adjustment is required during operation. Theextruded panel exits the die at approximately 79° C. A flat panel ofeven higher quality than Example 1 in terms of thickness variation andedge consistency is produced. The production rate is 5.8 m/hour (19feet/hr, or 71.4 kg/hour (160 lb/hr).

COMPARATIVE EXAMPLE 1

A commercial press molding operation is employed, employing a large,thick section coffin-like mold. UHWMPE resin powder is introduced intothe mold cavity and a close fitting top inserted into the mold. The moldand three other molds of similar size are loaded into a stack, placedunder a pressure of 1000 lb/in², (6.9 MPa) and heated to above thecrystalline melt temperature, held at this temperature for 90 minutes,and then slowly allowed to cool. Thick plaques are obtained from which0.375 inch (0.95 cm) thick panels can be produced by skiving in aseparate step. Prior to skiving, the production rate is 75 lbs/hour(33.5 Kg/hr). Following skiving, a long anneal time at elevatedtemperature is required to reduce end curl and edge curl and internalstresses.

EXAMPLE 3

A panel of UHMWPE of a nominal ⅜ inch (9.525 mm) thickness and a widthof about 1.2 m is ram extruded, as described in Example 2, from a diehaving a heated zone maintained at about 420° F. (215° C.) followed, inthe machine direction, by a plurality of cooling zones, such that a flatsheet having a thickness tolerance of ±0.030 inch can be produced duringstable operation. The sheet exits the die at a temperature of about 150°F. (65.6° C.), below the crystalline melt temperature.

The start-up temperatures are at a target temperature of 345° F. (174°C.). The hopper is filled with UHMWPE resin powder having a numberaverage molecular weight of about 4×10⁶ Da, and purged with nitrogen.The ram, having a thrust of 2 inches (5 cm) is cycled at the requiredinterval. As the material exits the die, it begins to warp and deform.Concurrently, it also bows hard to the left. To counter this distortion,a back pressure fixture is applied to the panel to regain and maintaincontrol.

The fixture consists of a bottom, cloth covered, rigid, flat surfaceback pressure fixture of plywood, positioned below the panel. Below thebottom back pressure fixture is a table supported from the floor. Thetable height is adjusted such that the top surface of the bottom brakefixture is close to the same level as the bottom of the slit dieopening. Three fixtures are constructed of rigid plywood covered withcloth, and placed on the top of the panel. The fixtures each measure 42″(1.07 m) in length and 16¾″ (42.5 cm) wide, and cover the first 42″(1.07 m) of panel across substantially the entire width. The top threefixtures are then pressed down against the panel with four pneumaticcylinders each, two cylinders on the left side of each fixture, and twoon the right, with the two on each side tied together and actuatedutilizing the same valving and regulator system. This combination givesthe ability to adjust each top brake fixture on both its left and rightsides, thereby creating six zones of pressure control across the widthof the part. The air cylinders are three inches in diameter, and can beregulated from 0 PSIG to 100 PSIG (0 bar to 6.9 bar gauge). Thecylinders are evenly spaced an approximately 26″ (66 cm) apart in themachine direction. On each side of the panel is a side guide device tokeep the panel from moving out from underneath the back pressurefixturing. These are actuated and controlled by similar valving,regulators, and cylinders as the top fixture, and are also made ofplywood.

The top fixtures are placed on the panel, and the regulators set at 10PSIG (0.69 bar gauge) to stop the warping. Once the fixture ispressurized, the sheet begins to flatten, but still bows to the left.Most of the warped areas are trimmed off, and the side of the panel istrimmed back so the edges of the panel are inside the side guide area.The side guides are then actuated at 20 PSIG (1.38 bar gauge),effectively encapsulating the panel. As the panel moves forward out ofthe brake fixture, it is guided between rollers guides for furthercontrol. The top rollers are initially raised 8″-10″ (20 cm-25 cm) abovethe bottom rollers to allow any sheet with warpage to easily passthrough. Once completely through the roller guides, the top rollers arelowered down onto the sheet. The roller guides are provided over alength of 20′ (6 m) and are furnished in pairs, one roller above and onebelow the panel. The rollers are 3″ (7.5 cm) in diameter, and spacedapproximately 5″ (1.25 cm) apart, center to center.

Once reasonably stabilized, the sheet is observed to have warping/raisedareas on the right side of the sheet. The back pressure zones are thenadjusted by raising the air pressure to the pneumatic cylinders to 40,40, 45, 50, 60, and 75 PSIG from left to right (2.8, 2.8, 3.1, 3.4, 4.1,5.2 bar gauge). This decreases, but does not completely stop thewarping, so the back pressure fixture is re-adjusted to 40, 40, 45, 50,75, 85 PSIG (2.8, 2.8, 3.1, 3.4, 5.2, 5.9 bar). At these settings, thesheet is no longer warped, but has a slight bow to the left. The backpressure is again adjusted to 40, 40, 50, 75, 80, 95 PSIG (1.8, 2.8,3.4, 5.2, 5.5, 6.6 bar). The sheet straightness now falls within therequired tolerance range, and no more adjustments are needed. Betweeneach adjustment, 30′ (9.1 m) of material is produced. This is due to thefact that the fixturing devices are 25′ (7.6 m) long, and as the sheetproduced under the new adjustments should extrude past the end of thefixture to assess its over all quality.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A ram extrusion process for preparing UHMWPE panels of widths of 0.4m or greater, comprising: providing a slit die having a width of 0.4 mor greater which has a substantially rectangular cross-section proximateits exit under ram extrusion operating pressure, the die having a topportion and a bottom portion defining a slit therebetween, and an inletface and an exit face, and having a plurality of independentlycontrollable transverse cooling zones in each of the top portion andbottom portion, said cooling zones located proximate the exit face ofthe die and positioned across the width of the die such that thetemperature of the cooling zones may be adjusted to vary the temperatureof respective portions of the slit die transverse to the exit face;incrementally introducing UHMWPE particles into the die under pressurefrom a ram; heating the UHMWPE particles to at least crystalline melttemperature of UHMWPE to form a substantially continuous moltenthermoplastic; cooling the UHMWPE to a temperature below the crystallinemelt temperature while within the die and while maintaining contact withwalls of the die; extruding from the slit of the slit die a solidifiedUHMWPE panel product, and restraining movement of the panel product awayfrom the die by a back pressure device to produce a back pressure withinthe die.
 2. The process of claim 1, wherein the temperature of the panelat its surfaces is between 40° C. and 100° C. upon exiting the slit die.3. The process of claim 1, wherein the slit of the slit die is notsubstantially rectangular as manufactured, further comprising distortingthe die to a substantially rectangular shape by applying ram extrusionpressure.
 4. The process of claim 1, wherein said back pressure devicecomprises a plurality of independently adjustable back pressure elementspositioned transverse to the machine direction along the width of thepanel such that the back pressure exerted at different positions acrossthe width of the panel can be differentially adjusted, the processfurther comprising adjusting the back pressure generated by respectiveback pressure element(s) relative to other back pressure generatingelements to maintain or increase at least one of panel flatness,thickness uniformity, and surface quality.
 5. The process of claim 1,wherein said slit die has minimally five cooling zones across the widthof the die proximate the exit face, on both top and bottom portions ofthe die.
 6. The process of claim 4, wherein said slit die has minimallyfive cooling zones across the width of the die proximate the exit face,on both top and bottom portions of the die.
 7. The process of claim 1,wherein the temperature(s) of respective cooling zone(s) are adjustedrelative to the temperature of other cooling zone(s) to maintain orincrease at least one of panel flatness, thickness uniformity, andsurface quality.
 8. The process of claim 1, further comprising sensingparallelism of the panel relative to a line parallel to the machinedirection, and applying a counteracting pressure against an edge of thepanel to counteract panel movement away from parallelism.
 9. The processof claim 8, wherein the counteracting pressure is applied at at leastone position proximate the location of the back pressure device alongthe machine direction.
 10. The process of claim 1, further comprising avertical movement restraining apparatus following the back pressuredevice, said apparatus bearing against the panel or spaced closelythereto, and which exerts restraint against vertical movement of thepanel or a portion thereof out of a plane substantially parallel withthe slit of the slit die.
 11. The process of claim 10, wherein thevertical restraining apparatus comprises a plurality of roll pairsextending across the width of the panel or a portion thereof andpositioned along the machine direction, with one roll of the roll pairspositioned above the UHMWPE panel and the other roll of the roll pairspositioned below the UHMWPE panel.
 12. The process of claim 10, wherethe length of the vertical movement restraining apparatus in the machinedirection is such that the panel has cooled to a temperature within therange of ambient to about 50° C. and lower than the temperature of thepanel upon exit from the slit die, by the time the end of the verticalmovement restraining apparatus is reached.
 13. The process of claim 1,wherein the shape of the slit of the slit die when not under pressure isdually tapered, the entrance slit of the slit die being of greaterheight than the exit slit, and being tapered in a vertical directionfrom the greater height to a lesser height in the machine directionbeginning at or after the furthest point of penetration of the ram intothe entrance slit; and a horizontal taper such that the height of theslit die at a central portion thereof in the width direction is lessthan the height at an edge portion thereof.
 14. The process of claim 4,wherein the shape of the slit of the slit die when not under pressure isdually tapered, the entrance slit of the slit die being of greaterheight than the exit slit, and being tapered in a vertical directionfrom the greater height to a lesser height in the machine directionbeginning at or after the furthest point of penetration of the ram intothe entrance slit; and a horizontal taper such that the height of theslit die at a central portion thereof in the width direction is lessthan the height at an edge portion thereof.
 15. The process of claim 5,wherein the shape of the slit of the slit die when not under pressure isdually tapered, the entrance slit of the slit die being of greaterheight than the exit slit, and being tapered in a vertical directionfrom the greater height to a lesser height in the machine directionbeginning at or after the furthest point of penetration of the ram intothe entrance slit; and a horizontal taper such that the height of theslit die at a central portion thereof in the width direction is lessthan the height at an edge portion thereof.
 16. The process of claim 6,wherein the shape of the slit of the slit die when not under pressure isdually tapered, the entrance slit of the slit die being of greaterheight than the exit slit, and being tapered in a vertical directionfrom the greater height to a lesser height in the machine directionbeginning at or after the furthest point of penetration of the ram intothe entrance slit; and a horizontal taper such that the height of theslit die at a central portion thereof in the width direction is lessthan the height at an edge portion thereof.
 17. The process of claim 1,further comprising restraining distortion of the slit die due to ramextrusion pressure by at least one support structure comprising top andbottom members located respectively above and below the top and bottomportions of the slit die, the top and bottom members tensioned with eachother.
 18. The process of claim 17, wherein said top and bottom supportmembers are tensioned by bolts connecting said members.
 19. The processof claim 18, wherein said bolts are multi-jackbolt tensioners.
 20. Theprocess of claim 1, further comprising both introducing UHMWPE powderfrom a hopper into an inlet of said slit die and removing oxygen fromsaid UHMWPE powder.
 21. The process of claim 20, wherein removing oxygencomprises sparging UHMWPE powder contained in the hopper with asubstantially oxygen-free gas.